In the experimental study of cardiac functions and in many clinical situations, it is often necessary to monitor the acoustic output of a patient's heart at the exterior surface of the chest cavity. Such measurements are generally difficult to make because the acoustic signals produced by the heart are very weak. Detection of these cardiac acoustic signals is often complicated by acoustic signals caused by motion of the patient and interference caused by airborne noise. In addition, ambient electromagnetic interference picked up by the patient's skin and by interconnecting cables can create spurious signals which interfere with the performance of the transducer.
Another common application for an acoustic biomedical transducer is in connection with the monitoring of a patient's respiration system. For this monitoring application, an acoustic transducer is placed in either the suprasternal notch or the supraclavicular region to detect acoustic signals corresponding to the patient's respiration. The transducer used in this type is generally similar to that used for cardiac system monitoring and, further, is susceptible to the same types of difficulties as those discussed above.
Previous transducers for monitoring acoustic signals related to cardiac functions and respiration have been ineffective in overcoming the difficulties discussed above. A common problem with previous transducers is related to their relatively light weight. Such transducers lack sufficient motional stability to avoid erroneous detection of cyclic movements of the patient's chest as an acoustic signal related to the functioning of the heart or respiratory system. Another common problem with prior transducers is that minute movements of the technician's hand can be erroneously detected as an acoustic cardiac or respiratory signal. Further, such transducers are often susceptible to detection of ambient acoustic signals which are transmitted through the rear of the transducer assembly.
An effective transducer for the applications described above must have a very high sensitivity to tissue-conducted acoustic signals across a broad spectrum, but must have a very low sensitivity to other acoustic signals, such as those caused by patient motion and airborne sound. Furthermore, the transducer must have a very low sensitivity to electromagnetic interference. In addition to the technical requirements discussed above, it is very important that the transducer be easily cleaned and sterilized. The prior art is lacking a biomedical transducer which meets the requirements discussed above.